Phenol-formaldehyde polymers formation reactions

Phenol - formaldehyde polymers are the oldest synthetic polymers. These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst. The reaction starts with the initial formation of o-and/or p-hydroxymethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through -CH2 groups. The initial product could be a linear product - Novolac used in paints. 

Benzylic cations have also been implicated in the formation of phenol-formaldehyde polymers. The synthesis of phenol-formaldehyde polymers, including the involvement of carbocations, has been reviewed [39], The involvement of benzylic cations in this reaction sequence is shown in Fig. 14. The first step of the acid-catalyzed reaction is the formation of a hydroxymethyl carbocation. Electrophilic substitution of phenol produces an o- or p-hydroxymethylphenol 91. Rapid protonation of the hydroxymethyl, followed by loss of H20, results in the formation of benzylic... 

As indicated previously, phenol-formaldehyde polymers find practical utilization mainly in the form of network polymers. The polymerization is normally carried out in two separate operations. The first operation involves the formation of a low molecular weight fusible, soluble resin and the second operation involves curing reactions which lead to the cross-linked product. Various types of initial low molecular weight resins are produced commercially and are considered in this section. 

In aqueous alkaline solution, phenol reacts with formaldehyde (methanal) at low temperatures to form a mixture of 2- and 4-hydroxy-benzyl alcohols. This hederer-Manasse reaction is another example of electrophilic attack which results in the formation of a new C-C bond. The mechanism is illustrated in Scheme 4.14. These products readily lose water to form quinomethanes (methylenecyclohexadienones), which react with more phenoxide. This process is repeated over and over again to produce a cross-linked polymer or phenol-formaldehyde resin (e.g. Bakelite) in which the aromatic rings are linked to methylene bridges. 

The formation of crosslinked networks has been the basis for polymer technology since the development of phenol-formaldehyde resins by Baekeland in 1910. The changes to the rheology of the phenolic system, as it develops from a liquid to a rubber and then to a glass, arise from the formation of a three-dimensional network as the step-growth chemical reactions occur. Water is evolved and the end product is a solid infusible mass. 

Fast pyrolysis of biomass produces a phenol-rich oil suitable for incorporation into phenol-formaldehyde (PF) resins. To evaluate the reactivity and network formation characteristics of the compounds typically found in these oils, a series of phenolic model compounds was reacted with formaldehyde under conditions typically used to prepare PF resins. This study indicates that the substituted phenolics commonly found in pyrolysis oils are more reactive than phenol. It also showed that the network formation process for tiiese complex phenolic mixtures follows the predictions of the simple statistical approaches developed by Flory (23) and Stockmayer (24). These results show that the substituted phenolics commonly found in pyrolysis oils will be chemically bonded into the polymer network and that under the proper reaction conditions a highly stable network will be formed. 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

The reaction of curing is not clear. It is known that under controlled conditions phenol and hexamethylenetetramine form a crystalline salt of the stiochiometric composition C6H12N4 SCeHsOH [30] which, when heated, evolves ammonia with the formation of an insoluble, infusible polymer [31]. In the presence of water, hexamethylenetetramine hydrolyzes with the formation of two moles of dimethylolamine DMA, one mole of formaldehyde and two moles of ammonia. Water is ubiquitious in novolacs and therefore under basic reaction conditions in the presence of tert and sec amines and also ammonia as shown in the chart, methylene bridges are formed by entering formaldehyde into the reaction. With increasing amounts of hexamethylenetetramine, the benzylamine type bridges become predominant. 

A classical example of tiie formation of a network polymer is the polycondensation of phenols with aldehydes. The reaction between phenol and formaldehyde in the absence of a catalyst is very slow and hence in all commercial s)mthesis, catalysts are always added to... 

The acid-catalyzed reaction occurs by an electrophilic substitution where formaldehyde is the electrophile. Condensation between the methylol groups and the benzene rings results in the formation of methylene bridges. Usually, the ratio of formaldehyde to phenol is kept less than unity to produce a linear fusible polymer in the first stage. Crosslinking of the formed polymer can occur by adding more formaldehyde and a small amount of hexamethylene tetramine (hexamine. 

The workhorse of the VLSI industry today is a composite novolac-diazonaphthoquinone photoresist that evolved from similar materials developed for the manufacture of photoplates used in the printing industry in the early 1900 s (23). The novolac matrix resin is a condensation polymer of a substituted phenol and formaldehyde that is rendered insoluble in aqueous base through addition of 10-20 wt% of a diazonaphthoquinone photoactive dissolution inhibitor (PAC). Upon irradiation, the PAC undergoes a Wolff rearrangement followed by hydrolysis to afford a base-soluble indene carboxylic acid. This reaction renders the exposed regions of the composite films soluble in aqueous base, and allows image formation. A schematic representation of the chemistry of this solution inhibition resist is shown in Figure 6. 

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